This invention pertains to the field of sewing machines, more particularly, to electronic controls for positioning sewing instrumentalities thereof.
In prior art electronically controlled sewing machines, the stitch position information is retained in a solid state memory, and release of bight and feed digital information therefrom is signalled by a machine synchronizer input at the appropriate times during each revolution of the horizontal arm shaft of the sewing machine. The information thus released is transferred to digital to analog converters, and the resulting analog signals are fed to servo systems to urge the sewing instrumentalities to the new positions related to the digital information. Thus, information may be retained in the solid state memory indicative of the extreme right hand position for a needle bar of the sewing machine, and the immediately subsequent stitch may request the needle bar to move to extreme left hand position. The positioning of the needle bar itself may be accomplished by a linear motor deriving its driving signal from the servo system connected to the digital to analog converter. The linear motor of the servo system is required to reposition the needle bar from one extreme position to the other extreme position in the approximately one-half revolution when the sewing needle attached to the needle bar has been elevated out of contact with the work material. For a sewing machine operating at 1200 RPM, it can be calculated that the linear motor is required to accomplish its objective in approximately 20 milliseconds. By proper design of the linear motor and the servo system, the maximum linear motor excursion time of approximately 20 milliseconds can be assured. However, at lower sewing machine speeds the impact sound of the linear motor can be more easily heard due to the relatively quiet running of the sewing machine. What is required is some means of reducing the acceleration of the linear motor, when possible, which would tend to reduce the impact forces in the mechanism and thereby quiet the sound during excursion. It is, of course, more desirable to reduce the acceleration of the linear motors to a greater degree at the lower speeds of the sewing machine, in as much as the impact noise of the linear motor is a greater proportion of the overall noise of the sewing machine at these lower speeds.